The present invention relates to a process for producing olefins from natural gas.
Natural gas is an abundant fossil raw material which is essentially composed of methane, the currently proved reserves of which amount to 10.sup.14 m.sup.3, which represents about 50 years of world consumption. Gas pools often show large amounts of ethane, propane, other superior alkanes as well as other constituents such as H.sub.2 O, CO.sub.2, N.sub.2, H.sub.2 S and He. The major part of the propane and other superior alkanes in the natural gas are liquefied and called LPG (liquefied petroleum gas). In pools which are rich in helium (generally more than 0.3% by volume), the helium is separated because of its high commercial value. The hydrogen sulfide is also separated because of its corrosiveness, and water is separated, because of the forming of hydrates which hinder the natural gas transportation. The natural gas obtained is then called a non-condensed gas and it mainly contains (for exemple 55-99% by volume) methane as well as 1 to 25% by volume of ethane and optionally low amounts of propane, nitrogen and/or carbon dioxide.
The major part of the natural gas is used for individual and industrial heating; still, there are some processes for converting natural gas into superior hydrocarbons.
Directly converting natural gas into ethylene would be highly desirable since ethylene can serve as a raw material for numerous syntheses of important products.
Two processes which allow reaching this goal are methane pyrolysis and methane catalytic pyrolysis, but these are very endothermic processes which require very high energy input. Moreover, both processes produce high, undesired amounts of coke.
Ethane pyrolysis is also a well-known, very endothermic process which is therefore a substantial power consumer. But, since ethane pyrolysis is achieved at temperatures that are lower than those used for methane, it is not possible to simultaneously convert these two compounds. So, when one operates at the ethane conversion temperature, the methane which is present in the ethane charge comes out of the reactor essentially unchanged.
Ethylene and other hydrocarbons can also be produced through the oxidizing coupling of methane, either in the sequential or in the simultaneous mode.
The reaction of oxidizing coupling in the sequential mode involves the oxidation of methane by a reducible agent, followed by the re-oxidation of said agent, which is carried out separately, by the oxygen in the air. Several U.S. patents (for example U.S. Pat. Nos. 4,499,323, 4,523,049, 4,547,611, 4,567,307) mention the use of numerous metal oxides, especially Mn, Ce, Pr, Sn, In, Ge, Pb, Sb, Bi, Tb as reducible agents for this reaction.
The reaction of oxidizing coupling in the simultaneous mode (scavenging of a mixture of methane and oxygen on a contact mass C) can be qualitatively expressed as: ##STR1##
The use of rare-earth oxides, alkaline and alkaline-earth oxides, and titanium, zirconium, hafnium and zinc oxides, alone or mixed, as catalysts for the reaction of oxidizing coupling in the simultaneous mode has been mentioned in several patents (for example European Patent Nos. EP 210,383 A2, EP 189,079 A1, EP 206,044 A1 and World Patent No. WO 86,07351).
Former efforts in the oxidizing coupling of methane have led to the forming of low ethylene/ethane ratios, generally ranging from about 0.8 to 1.2 for C.sub.2 + products selectivities higher than about 65%. Such low ethylene/ethane ratios require separating the ethane from the ethylene and carrying out the costly pyrolysis of ethane into ethylene. It should also be known that the former efforts in the oxidizing coupling of methane have not taken into account the other major constituents of natural gas such as ethane, propane and other saturated hydrocarbons. The processes concerning the oxidizing coupling of methane which are described above are not very suitable for the selective conversion of natural gas into olefins because of the presence of significant amounts of light hydrocarbons (such as ethane and propane) in natural gas. The relative oxidation rates of light alkanes under the conditions of oxidizing coupling of methane are about 15 to 100 times higher than those relative to the oxidation of methane, that is to say that the light hydrocarbons are converted into ethylene and carbon oxides before the methane begins to react. So, the former processes cannot be efficiently applied to natural gas.